1. Field of the Invention
The present invention relates to a power unit for a small vehicle, which is a unit including an internal combustion engine with a crankshaft, and also including a power transmission system, which is equipped with a continuously variable transmission controlled, for example, hydraulically, and which transmits the torque from the crankshaft to the drive-wheel side while reducing the speed of the torque.
2. Background of the Invention
A power transmission system disclosed in Japanese Patent Application Laid-Open Publication No. 2004-3425 is a conventionally known example of a power transmission system including an internal combustion engine and a continuously variable transmission. The configuration of this example includes an oil pump that generates hydraulic pressure when the pump operates ordinarily, and another oil pump that generates supplementary hydraulic pressure needed when the hydraulic pressure is low. The two oil pumps are positioned at places, with a space, apart from each other, and are driven independently by different drive mechanisms.
A configuration as is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-3425, that is, a configuration including two oil pumps which are positioned at places, with a space, apart from each other, and which are driven independently by different drive mechanisms, has the following problems. For the purpose of driving the two pumps, it is necessary to provide each one of these two pumps with its own component parts. In addition, there has to be a limitation on making the power unit compact. Therefore, when a power unit equipped with a plurality of pumps is to be mounted on, for example, a small vehicle, such as a motorcycle, the power unit, desirably, is made up of fewer component parts, and has a compact structure.